33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 205 



It would appear then that the bulk of the parental injected DNA per- 

 sists as large molecules which, as phage DNA synthesis begins, comprise a 

 decreasing portion of the DNA pool from which the DNA of mature parti- 

 cles is drawn at random. This hypothesis is in good accord with the observa- 

 tion that the earliest mature phages contain a larger fraction of parental 

 phage DNA atoms than phage made at a later time. 86, 87, 93 Evidence has 

 already been presented that a portion of the parental DNA transferred to 

 progeny is in large fragments while the remainder is relatively dispersed. 

 The full significance of these observations awaits clarification of the cur- 

 rent paradox concerning "stars" in T2 DNA. 



Current ideas of DNA replication favor the semiconservative 94 hypothe- 

 sis originally presented by Watson and Crick 95 whereby the two strands 

 of each DNA molecule are separated at each replication, while each individ- 

 ual strand is effectively immortal. 96 As previously indicated, Levinthal 53 

 has presented evidence that such a semiconservative replication can be 

 observed with "stars"; the 36% pieces of labeled phage are observed to 

 become approximately 20 % pieces in first generation progeny, and to con- 

 tinue as 20 % pieces during transfer to a second generation. Unfortunately, 

 there has been difficulty in the repetition of these experiments. Even if this 

 observation can be substantiated, it does not prove that all of the numerous 

 DNA replications taking place during phage multiplication are semicon- 

 servative. 



Very recently, application of the density labeling method, first applied 

 to the study of DNA replication in Escherichia coli by Meselson and Stahl, 96 

 has been made to the problem of T2 phage DNA replication. It has been 

 reported 97 that when density-labeled parental phage are grown in normal 

 bacteria, DNA molecules of hybrid density are observed among the first 

 generation phage progeny in accordance with the expectation of the Watson- 

 Crick hypothesis. 



b. Time Course of Synthesis of New Bacteriophage 



(1) Development of Viral DNA and Protein Pools. The presence of the 

 unusual pyrimidine component, 5-hydroxymethylcytosine, has made it pos- 

 sible to determine specifically the course of synthesis of the DNA of T-even 

 phages during infection independently of the concomitant changes in 

 amount and synthesis of the DNA of the host bacterium. This possibility 



93 A. D. Hershey, J. Gen. Physiol. 37, 1 (1953). 



94 M. Delbruck and G. S. Stent, in "The Chemical Basis of Heredity" (W. D. Mc- 

 Elroy and B. Glass, eds.), p. 699. Johns Hopkins, Baltimore, 1957. 



96 J. D. Watson and F. H. C. Crick, Nature 171, 964 (1953). 



96 M. Meselson and F. W. Stahl, Proc. Natl. Acad. Sci. U. S. 44, 671 (1958). 



97 A. W. Kozinski, quoted by W. Szybalski, Brookhaven Symposia in Biol. 12, 33 

 (1959). 



